Dimethyl carbonate (hereinafter ‘DMC’), a typical dialkyl carbonate, is colorless, odorless and has an environment-friendly molecular structure without any known toxicity to human body. Since DMC contains highly reactive groups of methoxy group, carbonyl group and carbonyl methyl oxygen group in its molecule structure, it can be used to replace highly toxic phosgene as carbonylating agent and also dimethyl sulfate and methyl halides as methylating agent.
DMC has an excellent solubility and is thus used as an environment-friendly solvent to replace halogenated solvents such as chlorobenzene. It has been widely used as a substitute for phosgene as a raw material for polycarbonate, an additive for improving the octane number of automotive fuel, and an electrolyte for rechargeable batteries.
DMC has been mainly synthesized by methanol and phosgene in the presence of highly concentrated sodium hydroxide solution. High toxicity of phosgene and corrosiveness of chlorine ion have been limiting a large-scale production of DMC and its applications.
In 1983, Enichem Company in Italy developed a non-phosgene method to synthesize DMC by oxidative carbonylation of methanol and carbon monoxide with oxygen in the presence of a monovalent copper chloride catalyst. However, this method has some problems such as use of a toxic carbon monoxide as a raw material, a low conversion rate and a high energy cost due to unreacted methanol and by-product water. Further, because the copper chloride (I) catalyst is readily oxidized to a divalent copper ion, its catalytic activity is reduced. Further, it also requires continued monitoring of the reaction chamber against the corrosion and explosion. In addition, due to the presence of a small amount of chloride ions in the product, the refining cost is considerably increased when DMC is used as an electrolytic solution in a secondary lithium battery.
Another conventional method for preparing DMC is Ube process. The process proceeds in two steps in gas-phase: in the first step, methanol reacts with nitrogen oxide (NO) and oxygen to give methylnitrite (MN) and water, without any catalyst. In the second step, MN reacts with carbon monoxide to produce DMC, in the presence of a palladium supported catalyst. In the catalytic process, the NO produced in the latter reaction is converted again to MN. Although the cost of energy for the separation and purification process is relatively low in this process, the use of the highly toxic and corrosive carbon monoxide and NO requires an anti-corrosion reaction chamber, an anti-explosion safety device for a precise controlling of raw materials concentration. Also, there is a problem that the reactants may leak.
Still another conventional method for preparing DMC is Texaco process in which ethylene oxide (or propylene oxide) and carbon dioxide are reacted with each other at high pressure in the presence of a catalyst to form ethylene carbonate (or propylene carbonate) and thus prepared DMC and ethylene glycol (propylene glycol) through ester interchange reaction with methanol Unlike the above-mentioned two conventional processes, the Texaco process does not use carbon monoxide and is thus considered a very safe process. However, since the process is performed at high temperature and pressure, there is still a possibility of explosion due to leakage of ethylene oxide. Moreover, the conversion rate is not very high, and thus it still requires a large amount of energy for the separation and purification of DMC and ethylene glycol as products from unreacted materials.
Yet still another method for preparing DMC is a method for directly synthesizing carbon dioxide and methanol at high temperature and pressure in the presence of a catalyst. However, the yield of DMC is extremely low in a thermodynamic equilibrium state
Recently, a method for preparing dialkyl carbonate by directly synthesizing urea and methanol in the presence of a catalyst has been actively studied. This method has the advantages that inexpensive urea is used as a raw material and, since water is not produced as a by-product, a ternary azeotropic mixture such as methanol-water-DMC is not formed, thus simplifying the separation and purification process. Moreover, the ammonia produced as a by-product can be reused by a urea formation by synthesizing ammonia with carbon dioxide, and thus it is possible to provide an environment-friendly process which does not produce by-products.
The methods for preparing DMC from urea and methanol are as follows. Method (1) for synthesizing DMC from urea and methanol in the presence of a zinc acetate catalyst (S. Bowden., E. Buther, J. Chem. Soc. 1939, vol. 78) and method (2) for synthesizing various dialkyl carbonates from urea and primary aliphatic alcohol in the presence of an organic metal compound catalyst such as magnesium methoxide [Mg(OCH3)2] and an organic phosphine catalyst such as triphenylphosphine (PPh3) (Peter Ball, Heinz Fullmann, and Walter Heintz, “Carbonates and Polycarbonates from Urea and Alcohol”, Angrew. Chem. Int. Ed. Engl. 1980, vol. 19, No. 9, pp 718-720, WO 95/17369). However, the above conventional methods (1) and (2) for preparing DMC have the problem that the yield is low.
Another a method (3) for preparing DMC in the presence of a catalyst complex comprising an organotin compound and a high boiling electron donor compound containing polyglycol ether such as triethylene glycol dimethyl ether (PGDE) is disclosed in U.S. Pat. No. 6,010,976 by J Yong Rye, and various process patents are disclosed in U.S. Pat. No. 6,392,078 B1 and U.S. Pat. No. 7,314,947 B2 based on method (3). However, the disclosed catalyst complex has the disadvantages that the catalytic activity is rapidly reduced by water contained in a raw material as an impurity and it has toxicity to the ecosystem. Moreover, the high boiling oxygen containing polyglycol ether compound used as a co-catalyst is decomposed or polymerized at high temperature, and thus the activity of the co-catalyst is reduced due to the change in viscosity by thermal decomposition. Further, the organotin catalyst and the polyglycol ether compound used as co-catalyst are to be discarded due to the difficulty in their recycling, and this raises an environmental issue.
There is another method (4) for preparing DMC by directly reacting urea with methanol in a catalyst rectification reactor or distillation column using alumina and silica supports on which metal oxides such as Zn, Pb, Mn, La, and Ce and alkali oxides such as K, Na, Cs, Li, Ca, and Mg are impregnated as reaction catalysts (U.S. Pat. No. 7,271,120). This method is an improved method that can easily separate a catalyst from a given product. However, because the reaction temperature is much higher than the boiling point of methanol, it is necessary to maintain the vapor-liquid equilibrium at high pressure. Moreover, if the produced ammonia and DMC) are not discharged, the reaction yield is reduced, and the amount of by-products such as N-methyl carbamate (N-MC) and N,N-dimethyl carbamate (NN-DMC) is increased due to a side reaction between methyl carbamate (MC) as an intermediate product and DMC. Therefore, in order to improve the reaction yield and distillation efficiency of DMC at the reaction temperature higher than the boiling point of methanol and under the high vapor pressure of methanol during the preparation of DMC by the reactive distillation. It is necessary to maintain the reaction temperature and the pressure at the vapor-liquid equilibrium and it is further necessary to discharge ammonia and distillate to obtain DMC.
Here, the distillate is obtained as an azeotropic mixture of DMC and methanol, and the concentration of DMC in the azeotropic mixture is reduced at high pressure, which reduces the productivity. Although the amount of by-products in method (4) is smaller than that of method (3), the amount of by-products such as N-MC and NN-DMC is increased due to a high reactivity of the synthesized DMC which is reacted with MC as an intermediate product at high pressure as represented by the following reaction scheme, which is well-known in the art (Yoshio Ono, “Dimethyl carbonate for environmentally benign reaction”, Pure & Appl. Chem., 1996, Vol. 68, No. 2, pp 367-375).

In addition, the method (5) for preparing DMC using polyethylene glycol dimethyl ether (PGDE, MW 250 to 270) as an organic solvent, which is stable at atmospheric pressure and reaction temperature, while inhibiting the decomposition of urea and MC in the presence of various metal catalysts is disclosed (Bolin Yang et al. “Synthesis of dimethyl carbonate from urea and methanol catalyzed by the metallic compounds at atmospheric pressure”, Catalysis communications, 2006, vol. 7, p. 472-477). PGDE used is an organic solvent used as a medium for maintaining the reaction temperature at atmospheric pressure and as an electron donor or used to inhibit the decomposition of raw materials. However, this method has also some problems such as recycling of used PGDE and catalysts due to decomposition, consumption during reaction and low yield per unit time.
Moreover, a method (6) for preparing dialkyl carbonate by the reaction of urea or alkyl carbamate and alkyl alcohol using a quaternary ammonium ionic liquid such as tetramethylammonium hydrogencarbonate methyl ester and tetramethylammonium carbamate and an organotin catalyst at a temperature of 160° C. and a pressure of 20 atmospheres is disclosed in U.S. Pat. No. 5,534,649. However, method (6) for preparing DMC using methyl carbamate, methanol, and an ionic liquid has the problem that the maximum yield of DMC is very low (4.13%).
In general, the reaction process of synthesizing dialkyl carbonate by the reaction of alkyl alcohol and urea can be represented by the following reaction scheme 1:

It can be seen from the above reaction scheme 1 that when the produced dialkyl carbonate and ammonia are effectively discharged from the reactor, the equilibrium reaction will shift to the forward direction, thereby increasing the reaction rate and yield. In the synthesis of DMC, due to the low boiling point of methyl alcohol as a reactant, it is necessary to increase the reaction pressure (15 to 25 atmospheres) in order to maintain the reaction temperature, and thus the solubility of dialkyl carbonate and ammonia produced at high pressure is increased. As a result, the equilibrium constant becomes low, thereby reducing the reaction rate and yield. Moreover, since the solubility of DMC produced at high pressure is also increased, the amount of undesired by-products such as N-MC and NN-DMC is increased.